Maturation of the hypothalamic--pituitary--gonadal axis in the male lamb: a review

We have studied the activity of the hypothalamic--pituitary--gonadal (HPG) axis in the male ovine fetus and newborn lamb. Circulating levels of gonadotropins, prolactin, cortisol (F), testosterone (T), dehydroepiandrosterone sulfate and delta 4androstenedione (delta 4A) were measured in fetal plasma in the third trimester of gestation and the testicular response to hCG was studied in ovine fetuses at 95 - 141 days. Ultradian variations of LH, FSH and testosterone and the testicular response to hCG also were assessed from 1 to 28 days postnatally. The data indicate that (1) fetal plasma LH, FSH, delta 4A and T levels are low from 95 days of gestation to term, while F levels increase in the last 10 days. Postnatally, F decreases rapidly. Spontaneous T peaks may occur as early as 36 hr of life. (2) Spontaneous LH and concomitant or subsequent T secretory peaks are observed by 3 days of age. (3) hCG can induce an increase in T production by interstitial cells in vitro and can increase the T testicular content and its release in the plasma from 95 days of intrauterine life. Desensitization to hCG also can be observed throughout the last trimester of gestation. We conclude that the HPG axis of the male lamb is active ty 36 - 72 hr of postnatal life and that the steroidogenic capacity of the ovine testis is developed several weeks prenatally. Consequently, the relative quiescence of the axis prenatally and in the first 24 hr of life seems to result from relatively low LH secretion and release, related to an undefined endogenous control, together with decreased Leydig cell sensitivity and relatively low enzyme activity limiting T release.